A magnetic compass comprises a compass card rotatably supported in a closed case having a transparent cover through which the card can be viewed. One or more permanent magnets on the card cause it to rotate so as to orient the card relative to terrestrial magnetic lines of force. The case of the compass is customarily filled with liquid which exerts a damping action on the card and also imparts buoyancy to the card so as to reduce the effective weight of the card on its bearing. The compass card is customarily supported on the tapered upper end of a shaft projecting up from the base of the case by a central bearing in the card having a downwardly opening conical recess in which the tapered upper end of the shaft is received. A bearing of this kind allows the compass card to rotate freely in the case and also allows it to tilt relative to the axis of the supporting shaft. Tilting is limited by engagement of the compass card with the case, for example with the cover which is usually close to the compass card both to provide good visibility of the card and also to prevent the bearing of the card from becoming unseated from the upper end of the shaft.
When a compass is used on a boat and in particular on a sailboat, the compass is subject to tilting. For example, the compass may be tilted 20 degrees or more by the heeling of the sailboat. As the compass card is supported above its center of gravity, it tends to stay level and is hence tilted relative to the case when the entire compass is tilted. This may cause a peripheral portion of the compass card to engage the case--for example the cover--whereupon rotation of the compass card is impeded by frictional engagement with the case. The compass may hence give an inaccurate reading.
To avoid this undesirable effect, magnetic compasses are sometimes mounted on gimbals so that the compass remains substantially level despite pitching, rolling or heeling of the boat. However, gimbal mounting of the compass considerably increases its cost and also results in a considerably bulkier instrument which may be difficult to accommodate and mount on a boat.